The process of the present invention relates to methods of producing syndiotactic vinyl aromatic polymers.
In the production of syndiotactic vinyl aromatic polymers such as syndiotactic polystyrene (SPS), a devolatilization finishing step is required to remove residual monomers, process solvents, and other volatile components from the SPS polymer. Residual vinyl aromatic and other monomers can autopolymerize upon heating to form atactic vinyl aromatic and other polymers which are undesirable contaminants in SPS polymers. The presence of atactic vinyl aromatic polymers in SPS polymers adversely affects product properties such as the heat distortion temperature and crystallization rate of the SPS homopolymers and copolymers.
JP 03056504 by Yamamoto discloses a melt devolatilization finishing step, wherein wet SPS powder containing volatiles is fed to a twin screw extruder, where it is melted and devolatilized by passage through one or more vacuum vents. However, extrusion devolatilization processes have limited capacity, higher equipment costs and are more difficult to scale to a commercial level when compared to solid state devolatilization processes.
JP 03064303 by Yamamoto discloses a solid state devolatilization finishing step, wherein wet SPS powder containing volatiles is predried at a temperature between the glass transition temperature, (typically about 100.degree. C.) and the melting point of SPS, and further devolatilized by melt devolatilization in a vacuum vented twin screw extruder as described in JP 03056504, above. However, Yamamoto discloses long residence times (9 or 10 hours) indicating low heating rates of the SPS powder which can result in the formation of significant quantities of atactic vinyl aromatic polymer.
Therefore, there remains a need for a method of removing volatiles and residual monomer(s) from syndiotactic vinyl aromatic polymers while minimizing the amount of atactic vinyl aromatic polymers produced.